1. Technical Field of the Invention
The present invention relates to an image processing system and a processing method thereof. In particular, the invention relates to an image-processing device including a solid-state image-pick up device provided with a plurality of unit pixels arranged in a matrix, each having a photo diode and a transistor for detecting a light signal, and a processing method thereof.
2. Description of the Related Art
A semiconductor image sensor is used for various kinds of image input devices. Recently, of such semiconductor image sensors, a MOS solid-state image-pick up device with a threshold voltage modulation system attracts attention since it provides high image quality attained by a CCD (charged coupled device) with low power consumption by a CMOS and restrains deterioration of an image quality and realizes high density and low cost.
For example, technology of a MOS solid-state image-pick up device with a threshold voltage modulation system is disclosed in the Japanese Patent Publication Laid-Open No. 11-195778. In a MOS solid-state image-pick up device with a threshold voltage modulation system, three states such as initialization, accumulation and reading out are repeated such that an image signal based on charges for emitting light that are accumulated in a carrier pocket of each pixel can be pulled out. An initialization period is a period of discharging residual charges from a carrier pocket. An accumulation period is a period of accumulating charges in a sensor cell. A reading out period is a period of reading a volume of charge out with voltage modulation.
However, according to the technology that has been disclosed so far, a frame rate was fixed and only a shutter speed was variable. The range of setting shutter speed (this is equivalent to an exposure period) is from 1H (horizontal period) to 1 frame.
Thus, the range of setting shutter speed is limited to 1 frame such that accumulation time can be insufficient when photographing in a dark environment and an image is brightened by boosting a signal gain of an amplifier. But, there is a problem of lowering the S/N ratio and deteriorating image quality.
On the other hand, when changing a framing rate, a system clock frequency is also changed. When the system is operated by reducing a frame rate, an H blanking period becomes long, when increasing a frame rate, an H blanking period becomes short. In addition, when a shutter speed is long, the frame rate is decreased. When a shutter speed is short (less than 1 frame rate), for example, a frame rate is 15 frames/second or 30 frames/second (it is referred to as fps hereafter).
Thus, when the system is operated by reducing the frame rate, an H blanking period becomes long, and it has a disadvantage of increasing noise due to dark current since a period of removing bias conditions in the accumulation state becomes long. In addition, if the system is operated with a high frame rate, a shutter speed is short and it has a disadvantage that photographing is limited in a bright environment to avoid increasing noise.
Thus, in view of the above issues, the present invention seeks to provide an image-processing device and a processing method thereof that improve a S/N ratio and contributes high image quality even in dark or bright environments.